Refers to someone you know, but not necessarily a close friend. Often used for people you have met or have a social connection with, but without the intimacy of 友人.
He is an acquaintance I met through work.
I joined the company through an introduction from an acquaintance.
He has an acquaintance in New York.
My friend introduced me to a business acquaintance.
I joined this company through an acquaintance's introduction.
I ran into an acquaintance at the station.